The present invention relates to mobile communication systems, and more particularly to methods and apparatuses for setting maximum power parameters at mobile communication system base stations having multiple antennas.
Multiple Input/Multiple Output (MIMO) processing is an advanced antenna technique for improving spectral efficiency and, thereby, boosting the overall system capacity of a telecommunication system. The use of MIMO processing implies that both the base station and the user equipment employ multiple antennas. There exist a variety of MIMO techniques (or modes). A few of these are: Per Antenna Rate Control (PARC), selective PARC (S-PARC), transmit diversity, receiver diversity, and Double Transmit Antenna Array (D-TxAA). The D-TxAA technique is an advanced version of transmit diversity that is used in the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN).
Irrespective of the applied MIMO technique, the notation (M×N) is generally used to represent MIMO configurations in terms of the number of transmit antennas (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2), and (8×4). Configurations represented by (2×1) and (1×2) are special cases of MIMO and correspond to transmit diversity and receiver diversity, respectively.
The above mentioned MIMO modes as well as other MIMO techniques enable various types of spatial processing to be applied to the transmitted and received signals. The ability to use spatial diversity in general improves spectral efficiency, extends cell coverage, enhances user data rate, and mitigates multi-user interference. In essence, each MIMO technique has its own benefit. For example, the receiver diversity technique (1×2) particularly improves coverage. By contrast, (2×2) MIMO techniques, such as D-TxAA, lead to increased peak user bit rates.
Although MIMO can be used to enhance the data rate, MIMO transmission also involves an increase in processing complexity and consumes more User Equipment (UE) battery power than non-MIMO transmissions. Therefore, MIMO processing is particularly feasible for high data rate transmissions. In UTRAN, high data rates are mapped onto the downlink shared channel (HS-DSCH). The embedded or in-band higher layer signaling, which may also be multiplexed on the HS-DSCH, could therefore be transmitted using MIMO.
By contrast, separate signaling or channels containing dedicated physical or higher layer signaling should preferably be transmitted using a conventional antenna technique (e.g., single antenna case). An example is UTRAN's use of an associated dedicated channel to run power control; sometimes this channel also carries higher layer signaling. Similarly, in soft handover the low bit rate dedicated channels could still be transmitted via one antenna.
The use of MIMO leads to significantly better performance compared to the baseline scenario of single transmit and receive antennas. But since a network may have to support both MIMO and non-MIMO user equipment, those user equipments supporting MIMO inform the network about their capability at the time of call setup or when doing registration processes. Certain technologies may support more than one MIMO mode. This means that, in one scenario, a particular base station may support all possible MIMO modes allowed by the corresponding standard while, in another scenario, the base station may offer only a sub-set of MIMO modes. In the basic arrangement, the base station may not offer any MIMO operation; that is, it may support only single transmit antenna operation. Therefore, the actual use of a particular MIMO technique is possible in scenarios when both the serving base station and user equipment bear the same MIMO capability.
There are two basic MIMO deployment scenarios: In a “MIMO only” scenario it is assumed that the serving base station, as well as all user equipments served by that base station, support the same MIMO technique, e.g. D-TXAA in case of UTRAN. This scenario is not very realistic because, in practice, there will almost always be low-end user equipments that do not support MIMO. However, it might be the case that, at times, all users in a cell have MIMO capability. At any given moment, the serving base station or the corresponding Radio Network Controller (RNC) in UTRAN will be fully aware of the multi-antenna capabilities of the user equipments it is serving. However, even when all users are MIMO capable, there might still be scenarios and occasions when the network may use single antennas for transmission of data and/or user-specific signaling. For example, low data rates could still be transmitted using single transmitted antennas. Also, congestion may force the network to use only single antenna transmissions even for high data rate services.
The second MIMO deployment scenario involves a mix of MIMO and non-MIMO users; that is, a mixture of users that are MIMO capable and those that only support the baseline configuration (i.e. single antenna transmission). This is a more realistic scenario. The baseline users (i.e., non-MIMO users) are likely either legacy users from earlier releases of the standard or are low end users.
In many densely populated areas, such as hotspots, an operator deploys more than one cell in the same geographical area (e.g., several cells in one sector). Each base station or Node B typically provides coverage to three sectors. As an example, a deployment with two carriers per Node B implies two co-located cells per sector and six cells per Node B. FIG. 1 is a schematic diagram of a Node B 100 in a UTRAN system. A user equipment 101 is representative of one or more user equipments that might be served by the Node B 100. The six so-called “co-located cells” are supported by the Node B's use of co-located carriers 103 which, in a UTRAN system, are each 5 MHz, as shown in FIG. 1.
A similar arrangement is conceivable in an evolved UTRAN (E-UTRAN) system. FIG. 2 is a schematic diagram of an eNode B 200 in an E-UTRAN system. A user equipment 201 is representative of one or more user equipments that might be served by the eNode B 200. The six co-located cells are supported by the eNode B's use of co-located carriers 203. Due to variable carrier frequencies in E-UTRAN, the co-located cells may have different bandwidths and, therefore, different maximum transmission power levels. The co-located carriers 203 having different bandwidths is shown in FIG. 2. However, even in E-UTRAN, the most common deployment scenario involves the co-located carriers 203 having the same bandwidth as one another.
In UTRAN systems, the co-located cells are likely to have the same maximum transmission power level. However, the value of the maximum transmission power level depends upon the base station class. For example, the maximum transmission power level in macro-cells can be 43 dBm, whereas in smaller cells (e.g. pico-cells), the maximum power budget is much lower (e.g. 24 dBm).
For E-UTRAN systems, in which the frequency bandwidth of a cell can be between 1.4 MHz to 20 MHz, the maximum cell power for a 20 MHz bandwidth can be up to 46 dBm in macro cells. By comparison, in cells having a smaller bandwidth, the maximum transmission power will be lower. The transmission in co-located cells will be served by multi-carrier power amplifiers (MCPA). An MCPA imposes limits on the maximum total transmission power per base station (or Node B or eNode B) as well as on the maximum transmission power per carrier (or co-locate cell). For convenience, the term “base station” is used throughout this specification and claims to denote not only traditional base stations, for example those employed in a system in accordance with Global System for Mobile communication (GSM) standards, but also Node Bs, eNode Bs, and any other equivalent node in a telecommunications system.
The total transmitted power per cell is limited. Therefore the maximum power available in a cell will be split between the transmitted antennas. If it is assumed that there are K co-located cells (or, equivalently, frequency carriers) and L antennas in a base station (e.g., Node B or eNode B) and that the maximum power setting per antenna for antenna “j” for a given carrier frequency “i” at a base station BS is denoted Pij, then these terms can be used to form a maximum base station power matrix, MmaxBS, for the base station, ‘BS’, on a linear scale. The maximum total base station power (PmaxBS) can be derived as follows:
      M    max    BS    =      [                                        p            11                                                p            12                                    K                                      p                          1              ⁢                                                          ⁢              L                                                                        p            21                                                p            22                                    K                                      p                          2              ⁢                                                          ⁢              L                                                            M                          M                          M                                                                                                  p                          K              ⁢                                                          ⁢              1                                                            p                          K              ⁢                                                          ⁢              2                                                L                                      p            KL                                ]  where each term pij(1≦i≦K and 1≦j≦L) can be considered to be a coefficient, cij times a maximum transmission power budget for a carrier i (Pmaxi).
Thus, the total maximum transmitted power of all the antennas for a particular carrier frequency ‘i’ can be expressed as
            ∑              j        =        1            L        ⁢          p      ij        =                    ∑                  j          =          1                L            ⁢                        c          ij                ⁢                  P          max          i                      =                  P        max        i            .      The total maximum transmitted power of all the antennas and of all the available carrier frequencies within the base station, ‘BS’, can then be expressed as
            ∑              i        =        1            K        ⁢          P      max      i        =            P      max      BS        .  The maximum transmission power in a base station will be set and maintained according to the equations above. However, these are general expressions that offer no guidance with respect to how to determine actual maximum transmission power settings. The settings used in state of the art technologies (e.g., UTRAN, E-UTRAN, etc) are described below.
The extent of cell downlink coverage is determined by the setting of common channel power levels. When MIMO is used at the base station the common channels (such as BCH, SCH, or channels containing pilot sequences) are generally transmitted from all or at least more than one antenna. However, their power settings can be different on different antennas. For instance, one of the antennas can be regarded as the primary antenna. The transmitted power of the common pilot sequence (e.g. as transmitted on the Common Pilot Channel—“CPICH”—in UTRAN) can be larger on the primary antenna than on any of the remaining antennas. For example, in the case of (2×2) MIMO, in a typical arrangement in UTRAN the CPICH power on the primary antenna can be twice that of the CPICH power set on the secondary antenna. This ensures good cell coverage of the non-MIMO users, which are generally served by the primary antenna.
The UE identifies cells and estimates the channel from the pilot sequences sent on the common channels (e.g., SCH, CPICH, etc.). Further, important radio resource functions like cell reselection, handover decisions, and the like, are also based on the measurements performed on the signals sent via the common channels. Therefore, in order to ensure consistent cell coverage, the power of the common channels on all the antennas remains fixed even if the maximum power per antenna is varied.
Regarding the UTRAN maximum power setting, the available transmission power budget per cell (i.e., PmaxC) is equally allocated among the multiple antennas. Since the same bandwidth (e.g., 5 MHz) is used in all of the co-located cells, the maximum base station transmission power matrix (MmaxBS) can be expressed as
      M    max    BS    =            [                                                                  P                max                C                            L                                                                          P                max                C                            L                                            K                                                              P                max                C                            L                                                                                          P                max                C                            L                                                                          P                max                C                            L                                            K                                                              P                max                C                            L                                                            M                                M                                M                                M                                                                              P                max                C                            L                                                                          P                max                C                            L                                            Λ                                                              P                max                C                            L                                          ]        .  
The value
      P    max    C    Lincludes the power of the common channels, MIMO users and non-MIMO users. As there are K cells per base station, the maximum total base station power (PmaxBS) can be expressed as PmaxC×K=PmaxBS.
To illustrate the point, for the case of (2×2) MIMO in a macro-cell environment and assuming two carrier frequencies per base station, the maximum base station power matrix can be represented as:
      M    max    BS    =            [                                    10                                10                                                10                                10                              ]        .  
Regarding the E-UTRAN maximum power setting, the available transmission power budget per cell in cell “i” (i.e. Pmaxi) is also allocated equally among the multiple antennas. However, the maximum power per cell within the same base station (e.g., eNode B) may be different for the different cells if they have different carrier bandwidths from one another. In case the same bandwidth is used in all the co-located cells, the maximum base station power matrix (MmaxBS) will be the same as that set forth above for the case of UTRAN. However, if different carrier bandwidths are used in the co-located cells, then the maximum base station power matrix (MmaxBS) will be given by
      M    max    BS    =            [                                                                  P                max                1                            L                                                                          P                max                1                            L                                            K                                                              P                max                1                            L                                                                                          P                max                2                            L                                                                          P                max                2                            L                                            K                                                              P                max                2                            L                                                            M                                M                                M                                M                                                                              P                max                K                            L                                                                          P                max                K                            L                                            Λ                                                              P                max                K                            L                                          ]        .  
As before, each component
      P    max    i    Lof the matrix includes the power of common channels, MIMO users and non-MIMO users. As there are K cells per base station, the maximum total base station power (PmaxBS) can be expressed as
            ∑              i        =        1            K        ⁢          P      max      i        =            P      max      BS        .  
To illustrate this with an example, for the case of (2×2) MIMO with two carriers per base station (e.g., eNode B) used in macro-cellular environment and assuming that carrier#1 and carrier#2 have bandwidths of 10 MHz and 20 MHz, respectively, the corresponding maximum power budgets per carrier for carrier#1 and carrier#2 are 40 W and 20 W respectively. The total maximum power per antenna is thus
      M    max    BS    =            [                                    20                                20                                                10                                10                              ]        .  
For both UTRAN an E-UTRAN, it is the case that the base station can make full use of the base station transmitted power resources only if all users served by the same base station support MIMO and if all of these users are served by using the full MIMO capabilities of the UE and the serving base station. However, in practice, it is unlikely that these conditions will often be satisfied because it is highly probable that there will be a mixture of MIMO and non-MIMO users (using single transmit antenna) in a cell whereof the latter users will be served by the primary antenna. Secondly even if all users are MIMO capable, some of them may not be served with all possible antennas all the time. For at least these reasons, the strategy of allocating a maximum transmitted power budget equally among multiple antennas is not optimal.
It is therefore desired to have methods and apparatuses that allocate maximum transmitted power budgets among multiple base station antennas in a way that allows the base station to make better use of its total transmitted power resources.